US5309126A

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Publication number: US5309126A
Application number: US07/793,925
Authority: US
Inventors: Donald E. Allen
Original assignee: Motorola Inc
Current assignee: CTS Corp
Priority date: 1991-11-18
Filing date: 1991-11-18
Publication date: 1994-05-03
Anticipated expiration: 2011-11-18
Also published as: US5463361A; DE69220894D1; KR100266243B1; EP0543556B1; DE69220894T2; EP0543556A1; JPH05267989A

Abstract

An electronic apparatus for receiving and filtering high frequency waves comprises a sequence of T electroacoustic transducers each having multiple interdigital electrode fingers and a centerline located equidistant between its outermost electrode fingers. For a centerline-to-centerline distance of a first adjacent pair of transducers D₁, and centerline-to-centerline distances of other adjacent pairs of transducers D₂ to D_T-1, then at least some of D₂ to D_T-1 differ from D₁ by a non-integral number of wavelengths. The transducer array is symmetrical about a centerline and the number of fingers in each transducer varies, with an input or output transducer closer to either end of the sequence having more fingers than a like kind transducer nearer the center.

Description

FIELD OF THE INVENTION

The present invention concerns an improved means and method for surface acoustic wave filters and apparatus using such filters.

BACKGROUND OF THE INVENTION

Modern portable radios and other electronic equipment require filters of very compact form and good electrical properties. Surface acoustic wave (SAW) filters are especially desirable for such purposes.

SAW filters are known in the prior art. For example, an article by Meirion Lewis entitled, "Saw Filters Employing Interdigitated Interdigital Transducers, IIDT", Ultrasonics Symposium, 1982, pages 12-17, describes SAW filters using interdigital transducers of interdigital electrodes on piezo-electric materials, each transducer having the same number of electrode fingers. Further, U.S. Pat. No. 4,492,940 to Hikita describes an acoustic surface wave bandpass filter using a linear array of alternate I/O transducers arranged in sequence, each having multiple interdigital electrode fingers, where the number of fingers in the input transducers and the number of fingers in the output transducers decreases from the center to the end of the array. Additionally, U.S. Pat. No. 4,649,357 to Nagai et al., describes a surface acoustic wave filter using a linear array of alternate I/O transducers arranged in sequence, each having multiple interdigital electrode fingers, where the distances (along the array) between the centerlines of adjacent transducers differ by an integral multiple (0, ±1, ±2, etc.) of wavelengths, and U.S. Pat. No. 4,931,755 to Sakamoto et al., describes use of an auxiliary capacitive transducer or other shunt capacitance to adjust the filter cut-off above the pass-band. The above-listed patents are incorporated herein by reference.

Despite the considerable effort by many researchers, numerous problems remain in connection with SAW filters, as for example, setting the optimum input/output (I/O) impedance levels, optimizing the spectral response, reducing internal reflections within the filter, and simplifying the fabrication process. These features of SAW filters affect their utility and performance in electronic apparatus. Hence, there continues to be an ongoing need for improved filters and filter applications.

As used herein the abbreviation SAW is intended to stand for "surface acoustic wave". As used herein the word "filter", singular or plural, is intended to include any element having a frequency dependent transfer function, and the words "SAW filter", singular or plural, are intended to refer to elements employing surface acoustic waves and having a frequency dependent transfer function.

SUMMARY OF THE INVENTION

The present invention provides an improved means and method for electronic devices and apparatus including SAW devices, and especially SAW filters and improved apparatus based thereon.

According to a first embodiment of the present invention, there is provided an electronic apparatus comprising a SAW filter including, in general terms, a substrate for propagating acoustic waves on which are arranged in a direction of wave propagation, a sequence of input transducers and output transducers, each transducer having a number of interdigitated electrode fingers, wherein beginning at one end of the sequence, the number of fingers in a first transducer is larger than the number of fingers in another transducer further along the sequence.

According to another embodiment of the present invention, there is provided an electronic apparatus comprising a SAW filter having a centerline-to-centerline separation of an adjacent pair of transducers of the SAW filter which differ from a centerline-to-centerline separation of another adjacent pair of transducers by a non-integral number of wavelengths of the acoustic wave.

In a preferred embodiment of the SAW filter, beginning at one end of the sequence, the number of fingers in a first input (or output) transducer is larger than the number of fingers in another input (or output) transducer further along the sequence. The sequence is desirably symmetric about a centerline normal to the direction of propagation of the acoustic wave. It is further desirable that where a transducer in the sequence contains an even number of electrode fingers, an adjacent transducer contains an odd number of electrode fingers.

There is further provided a method for forming an apparatus including a SAW filter which comprises providing a substrate for propagating acoustic waves and providing on the substrate a sequence of input transducers and output transducers, wherein each transducer has a number of interdigitated electrode fingers, wherein beginning at one end of the sequence, the number of fingers in a first transducer is larger than the number of fingers in another transducer located further along the sequence.

In a preferred embodiment of the method, the step of providing the transducers comprises providing transducers having predetermined centerlines normal to the direction of acoustic wave propagation, wherein each centerline is located half way between outer edges of first and last electrode fingers of the transducer, and wherein centerlines of a first adjacent pair of transducers are a distance D₁ apart and centerlines of other adjacent pairs of transducers are distances D₂ to D_T-1 apart. Here, T is the number of transducers and at least one of D₂ to D_T-1 differs from D₁ by a non-integral number of wavelengths.

It is further desirable that the step of providing the transducers comprise providing an input transducer with an even number of electrode fingers and another input transducer with an odd number of electrode fingers, and providing an output transducer with an odd number of electrode fingers, or vice versa as regards the designations of input and output.

In a still further embodiment, there is further provided an improved radio having therein one or more of the above-described filters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a highly simplified schematic drawing of the transducer arrangement of a first SAW filter according to the prior art;

FIG. 2 is a highly simplified schematic drawing of the transducer arrangement of another SAW filter according to the prior art;

FIG. 3 is a highly simplified schematic drawing of the transducer arrangement of a SAW filter according to an embodiment of the prsent invention;

FIGS. 4 and 5 are plan views illustrating transducers such as are used in the present invention, having odd and even numbers of electrode fingers, respectively, and showing the locations of the centerlines thereof;

FIG. 6 is a plan view illustrating multiple transducers such as are used in the present invention, arranged in a sequential array and illustrating the relationship between adjacent transducers, the centerlines thereof and the direction of propagation of the acoustic wave; and

FIG. 7 shows a simplified schematic diagram of a radio employing multiple filters, as for example, one or more SAW filters according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a highly simplified schematic drawing of the transducer arrangement ofSAW filter 20 according to the prior art as described, for example, by Nagai, supra.Filter 20 comprisesinput terminal 21 connected to a plurality ofinput transducers 22 andoutput terminal 23 connected to a plurality ofoutput transducers 24. Each

transducer

22, 24 is composed of a number of interdigital metal fingers coupled respectively to input 21 andoutput 23 and toground 26.Input transducers 22 andoutput transducers 24 are themselves interdigital. Thus,filter array 20 is formed by a sequence ofalternating input 22 andoutput 24 transducers, eachoutput transducer 24 having the same number of fingers and each being spaced from the

other transducers

22, 24 by distances l_i =l₁ to l_T-1, where T is the number of transducers.. Nagai teaches that it is desirable that the distances l_i be related such that for arbitrary distance l₁ =m₁ ·L where m₁ is a positive real number and L is the acoustic wavelength, then the other distances l₂ to l_T-1 satisfy l_i =l₁ +m_i ·L where m_i is an arbitrary integer (0, ±1, ±2, etc.), for i=2, 3, 4, . . . T-1. Thus, the distances l_i must be equal or differ by an integral number of wavelengths L.

FIG. 2 is a highly simplified schematic drawing of the transducer arrangement of another priorart SAW filter 30 according to Hikita, supra.Filter 30 hasinput terminal 31 connected to a plurality of

input transducers

32, 33, 34 symmetrically arranged around thefilter centerline 39.Filter output 35 is connected to a plurality of

output transducers

36, 37 which are also grouped symmetrically aroundfilter centerline 39. The transducers are also connected toground lead 38. The interdigital array of transducers makesfilter 30.

Hikita teaches that the number of electrode fingers in the transducers desirably taper as one moves from transducers located at or near the centerline toward transducers located at or near the ends offilter 30. That is, if N₁, N₂, and N₃ are the number of fingers respectively in

input transducers

32, 33, 34 and M₁ and M₂ are the number of electrode fingers in

output transducers

36, 37 respectively, then N₁ ≧N₂ ≧N₃ and M₁ ≧M₂.

While the above described prior art arrangements provide filtering action, they still suffer from some disadvantages well known in the art. For example, power handling capability is limited, it is difficult to optimize I/O impedance levels, and the spectral response of the filter is difficult to adjust. For this reason, such filters are still difficult to make and use or have less than the desired performance in electronic circuits, especially radios.

The foregoing and other problems are ameliorated or overcome by the structure and method of the present invention which is illustrated in various forms in FIGS. 3-7. Unless specifically noted otherwise, the designations "input" and "output" (and the abbreviation "I/O"), whether singular or plural, with respect to the present invention are intended to be arbitrary designations for purposes of illustration and discussion, and can be interchanged.

Referring now to FIG. 3 which is a highly simplified schematic drawing of the transducer arrangement ofSAW filter 40 according to an embodiment of the present invention.Filter 40 comprisesinput 41 coupled to a plurality of

transducers

42, 42', 43, 43', 44 and 44' having numbers of interdigital electrode fingers N_i denoted N₁, N₁ ', N₂, N₂ ', N₃ and N₃ ', respectively.Filter 40 further comprisesoutput 45 coupled to a plurality of

transducers

46, 47, 47', 48, 48', 49 and 49' having numbers of interdigital electrode fingers M_j denoted M₁, M₂, M₂ ', M₃, M₃ ', M₄ and M₄ ', respectively. Transducers 42-44' and 46-49' are arranged in a sequential array, so that the input-output transducers desirably alternate as one progresses along the array. For example, starting at one end, a sequence of

output transducers

49, 48, 47, 46, 47', 48' and 49', alternate with a sequence of

input transducers

44, 43, 42, 42', 43' and 44' to form the sequence output-input-output-input-output-input- etc., (e.g.,

transducers

49, 44, 48, 43, 47, 42, etc.) making up the exemplary SAW device.

As indicated on FIG. 3, the transducer's electrode "fingers" are interdigitated or interleaved and desirably perpendicular todirection 52 of propagation of the acoustic wave. For a given transducer, the electrode fingers facing in a first direction are coupled to the input (or output) and the remaining electrode fingers of that transducer (facing in the opposite direction) are coupled to a common terminal, as forexample ground connection 50.

The transducers themselves are also interleaved or interdigitated, that is, the ground contacts to the transducers are on one side of the array for the input transducers and on the other side of the array for the output transducers and the conductors leading to the input terminal approach only from one side of the array and the conductors leading to the output terminal approach from the other side of the array. This is not essential but is desirable and provides a convenient conductor layout.

Further, the transducers offilter 40 are desirably but not essentially symmetrical aboutcenterline 51 passing through the center offilter array 40 perpendicular toacoustic wave direction 52. For convenience of explanation and without intending to be limiting, symmetry aboutcenterline 51 is assumed in the discussion that follows and reference to transducers 42-49 is intended to include transducers having corresponding primed reference numbers.

The number of electrode fingers N_i, M_j in the transducers desirably varies according to the position of the transducer in the transducer array. It has been found that excellent electrical properties are obtained when the numbers of electrodes in the transducers varies such that the outermost transducers have more electrode fingers than the inner transducers. Starting at one end of the array and progressing towardscenterline 51, the number of fingers in the first input (or output) transducer is larger than the number of fingers in the next encountered input (or output) transducer. In a preferred embodiment the number of fingers N_i, M_j in the outermost input or output transducer is also larger than the number of fingers in the most centrally located input or output transducer. For example, N₃ ≧N₂ and/or M₄ ≧M₃, and preferably, N₃ ≧N₁ and/or M₄ ≧M₁.

The following is an example of a filter having thirteen transducers (T=13) with a center frequency of about 922 MHz, a pass-band of approximately 35±2 MHz with stop-bands on either side, a pass-band insertion loss of about 3 db and a stop-band rejection of at least 20 db. The exemplary filter is symmetrical aboutcenterline 51. The number of electrode fingers n in the various transducers is shown in Table I below. As an aid to relating this example to the transducers in FIG. 3, the reference numerals of the transducers of FIG. 3 are shown in parenthesis.

              TABLE I                                                     ______________________________________                                    NUMBER OF ELECTRODE FINGERS                                               IN DIFFERENT TRANSDUCERS                                                  TRANSDUCER         n      TRANSDUCER     n                                ______________________________________                                    N.sub.1 (42,42')                                                                       =     61     M.sub.1 (46)                                                                         =   45                               N.sub.2 (43,43')                                                                       =     55     M.sub.2 (47,47')                                                                     =   40                               N.sub.3 (44,44')                                                                       =     65     M.sub.3 (48,48')                                                                     =   40                                                         M.sub.4 (49,49')                                                                     =   48                               ______________________________________

The numbers of fingers in the primed and unprimed transducers are equal in this example. This is preferred.

It is also desirable that the distance between the centerlines of adjacent pairs of transducers be adjusted so as to not differ by an integral multiple of acoustic wavelengths. This is explained below in more detail.

As shown on FIG. 3, adjacent pairs of transducers, e.g., 49/44, 44/48, 48/43, 43/47, 47/42 and 42/46, are separated by transducer centerline-to-centerline distances d_i, which are in general not equal. Where T is the total number of transducers inarray 40, the distances d_i =d₁, d₂, d₃, . . . d_T-1 for i=1, 2, 3, . . . T-1 are not necessarily equal. With respect to the parameter d, the index "i" is intended to be arbitrary in the sense that any of the distances d_i can be assigned as i=1, and any other as i=2, and so on without a particular order or sequence being implied. Determination of the locations of the centerlines of the transducers is explained more fully in connection with FIGS. 4-6.

FIG. 4 is a planview illustrating transducer 60 having an odd number of electrode fingers therein.Electrode fingers 62 extend inwardly in a first direction from firstcommon connection 64 andelectrode fingers 63 extend inwardly in the opposite direction fromcommon connection 65.

Fingers

62, 63 are interdigital, that is, they interleave.

Electrode fingers

62, 63 are typically one-quarter wavelength wide and are typically spaced apart by about one-quarter wavelength so that an electrical potential can be created therebetween.Transducer 60 haswidth 66 extending betweenouter-most edges 621, 621' offingers 622, 622' offinger group 62 extending fromconnection 64.Transducer 60 hascenterline 67 which is normal to wavepropagation direction 52.Centerline 67 is half way between the outermost edges of

fingers

621, 622, that is, the

distances

68, 68' are equal. It will be noted that for a transducer with an odd number of fingers,centerline 67 falls on an electrode finger.

The number n of electrode fingers intransducer 60 is defined as the number of fingers extending fromcommon connection 64 plus the number of electrode fingers extending fromcommon connection 65. Whiletransducer 60 is shown as having n=17, this is merely for convenience of illustration and not intended to be limiting and larger or smaller numbers of fingers may be used, as is shown for example in Table I above.Transducer 60 is intended to illustrate a convenient transducer having multiple electrode fingers where n is an odd number.

FIG. 5 is a planview illustrating transducer 70 having an even numbers of electrode fingers therein. Electrode fingers 72 extend inwardly in a first direction from firstcommon connection 74 andelectrode fingers 73 extend inwardly in the opposite direction fromcommon connection 75.Fingers 72, 73 are interdigital, that is, they interleave.Electrode fingers 72, 73 are typically one-quarter wavelength wide and are typically spaced apart by about one-quarter wavelength so that an electrical potential can be created therebetween.Transducer 70 haswidth 76 extending between

outer-most edges

721, 731 of

fingers

722, 732.Finger 722 belongs to finger group 72 extending fromcommon connection 74 andfinger 732 belongs to fingergroup 73 extending fromcommon connection 75.

Transducer 70 hascenterline 77 which is normal to wavepropagation direction 52.Centerline 77 is half way between the

outermost edges

721, 731 of

fingers

722, 732, that is, thedistances 78, 78' are equal. It will be noted that for a transducer with an even number of fingers,centerline 77 falls between electrode fingers inspace 79.

The number n of electrode fingers intransducer 70 is defined as the number of fingers extending fromcommon connection 74 plus the number of electrode fingers extending fromcommon connection 75. Whiletransducer 70 is shown as having n=16, this is merely for convenience of illustration and not intended to be limiting and larger or smaller numbers of fingers may be used, as is illustrated for example in Table I above.Transducer 70 is intended to illustrate a convenient transducer having multiple electrode fingers where n is an even number.

FIG. 6 is a plan view illustrating multiple transducers such as are used in the present invention, arranged in asequential array 80 and illustrating the relationships between

adjacent transducers

60, 70, 60', the centerlines thereof 67, 77, 67' anddirection 52 of propagation of the acoustic wave. FIG. 6 showstransducer array 80 making up a portion of filter 40 (FIG. 3).Array 80 comprises evennumber electrode transducer 70 sandwiched between odd-number electrode transducers 60, 60', such as are shown for example in FIGS. 3-5. Transducers 60 (odd), 70 (even), 60' (odd) in FIG. 6 are intended to exemplify various of the transducers in the filter of FIG. 3 having various numbers n of electrode fingers which are not necessarily the same. For example,

transducers

60, 70, 60' of FIG. 6 exemplify transducers 42 (e.g., n=61), 47 (e.g., n=40) and 43 (e.g., n=55) respectively and in either order, or transducers 43 (e.g., n=55), 48 (e.g., n=40) and 43 (e.g., n=65) respectively and in either order, or their primed counterparts. Corresponding to the arrangement of FIG. 3,

connections

64', 75, 64 indicated by the letter "G" are coupled to common orground lead 50 offilter 40, andconnection 74 tooutput 51 andconnections 65', 65 to input 41 offilter 40.

Transducers 60, 60' havewidths 66, 66' andcenterlines 67, 67' defined in the same manner as for FIG. 4, that is, distances 680 and 681 are equal and distances 680' and 681' are equal.Transducer 70 haswidth 76 andcenterline 77 defined in the same manner as for FIG. 5, that is, distances 780 and 781 are equal.

Centerlines

67, 77, 67' are separated by

distances

81 and 82 respectively. Analogous distances toother transducers 700, 700' lying to the right and left respectively, oftransducers 60, 60' are denoted as 82' and 81' respectively. In FIG. 6transducers 700 and 700' are illustrated as being further transducers with n even. However, this is merely for convenience of illustration and not intended to be limiting, since adjoining further transducers may have n even or odd.

Distances

81', 82, 81, 82' illustrate the manner in which distances d_i in FIG. 3 are intended to be measured, that is, between the centerlines of the respective adjacent transducers. In the exemplary 13 transducer filter described earlier in connection with Table I and FIG. 3, the preferred centerline-to-centerline (CTC) distances are typically as shown in Table II:

              TABLE II                                                    ______________________________________                                    TRANSDUCER PAIR                                                           REFERENCE NUMBERS   CTC DISTANCES d.sub.i                                 ______________________________________                                    46-42               m.sub.1 · L + x                              42-47               m.sub.2 · L + x - L/4                        47-43               m.sub.3 · L + x + L/4                        43-48               m.sub.4 · L + x - L/4                        48-44               m.sub.5 · L + x + L/4                        44-49               m.sub.6 · L + x - L/4.                       ______________________________________

The CTC distances for the primed reference numbers are the same. The quantity x is a design parameter and a positive real number. The quantity L is the acoustic wavelength and m₁. . . m₆ are integers. Values of x about x=0.4 L are suitable, with values in the range of about 0.2 L to 0.6 L being useful.

Useful values of m₁ . . . m₆ are determined by the number of fingers in adjacent transducers and their separation. There must be sufficient space between centerlines to at least accommodate the numbers of fingers in the adjacent transducers. Typically, the finger width and finger separation in the direction of wave propagation are about L/4 so that the distance from the leading edge of one finger to the leading edge of the next finger is about L/2. The larger the number n of fingers, the larger must be the CTC spacing of adjacent transducers. Hence, minimum values of m₁ . . . m₆ are related to d_i and n. Those of skill in the art will understand how to determine such minimum values. In general, it is desirable that m₁ . . . m₆ be close to or equal to the minimum values.

As shown in Table II, at least some of the CTC distances d_i of the preferred embodiment of the present invention differ by non-integral multiples of the acoustic wavelength L, irrespective of which distance d_i is chosen as a base for comparison.

In FIG. 6, the "ground" of "common" terminals of the transducers lie on opposite sides of the longitudinal axis (parallel to propagation direction 52) offilter 80. These ground terminals are desirably all connected together by hooking the last ground finger of one transducer to the oppositely lying ground electrode of the next transducer. For example,left-most finger 622 oftransducer 60, which extends from groundpotential electrode 64, is connected bylongitudinal conductor 751 to "ground"potential electrode 75 oftransducer 70. Similarly, right-most electrode 622' which extends from "ground" potential electrode 64' of transducer 60' is connected to "ground"potential electrode 75 oftransducer 70 bylongitudinal conductor 752.

FIG. 7 is a simplified schematic diagram indicating how the above-described filters according to the present invention are advantageously used inradio circuit 90.Radio circuit 90 comprisesantenna 91 sending or receiving signals from optional diplexer 92 (which is coupled to the transmitter if one is used).Diplexer 92 sends an incoming signal to filter 93 which passes the resulting band limited signal toamplifier 94. Fromamplifier 94 the signal passes through anotherband limiting filter 95 tomixer 96.Mixer 96 also receives a signal fromlocal oscillator 97 throughband limiting filter 98. The mixed signal frommixer 96 passes throughband limiting filter 99 which removes the local oscillator signal and sends the result to the receiver IF viaoutput 100.

Filters

93, 95, 98 and/or 99 are advantageously SAW filters of the type described here and made according to the structure and method of the present invention, but of varying frequency or other properties according to the particular desired function. For example, filter 93 removes input RF frequencies outside the band in which the receiver is intended to operate. This is particularly useful for narrow band receivers such as are frequently required in cellular phone and paging applications and the like.

Filter 95 may have the same or a different pass band thanfilter 93 and removes any undesired harmonics generated byamplifier 94 or other out-of-band signals not removed byfilter 93.Filter 98 desirably passes the LO frequency and stops undesired harmonics thereof.Filter 99 desirably passes the sum or difference frequencies produced bymixer 96, and blocks the local oscillator and input RF frequencies. This is important to avoid saturating the input stages of the IF amplifier which is typically coupled tooutput 100. Thus, electronic apparatus, and especially radios have improved properties as a result of the improved properties of the SAW filters of the present invention.

The above-described filters are constructed by a method comprising, in a first embodiment, providing a substrate for propagating acoustic waves and providing on the substrate a sequence of input transducers and output transducers, each transducer having a number of interdigitated electrode fingers, wherein the a number of fingers in a first input or output transducer at or near a first end of the sequence is larger than the number of fingers in another input or output transducer more remote from the first end. Lithium niobate, quartz and lithium tantalate are examples of suitable substrate materials, with the latter being preferred. The transducer are formed by depositing a conductive film, usually a metal such as aluminum, on the substrate and then by means of photo-masking and etching steps well known in the art, the unwanted portions of the metal film are removed. These metallization, masking and etching steps are conventional and well known in the art. What is different is the shape of the electrode pattern formed in the film. This is accomplished by providing an etch mask having an image of the electrode shape and arrangement described herein.

In a further embodiment there is provided a method comprising, providing a substrate for propagating acoustic waves and providing on the substrate a sequence of transducers having predetermined centerlines normal to the direction of acoustic wave propagation, wherein each centerline is located half way between outer edges of first and last electrode fingers of each transducer, and wherein centerlines of a first adjacent pair of transducers are a distance D₁ apart and centerlines of other adjacent pairs of transducers are distances D₂ to D_T-1 apart, where T is the number of transducers, and wherein at least one of D₂ to D_T-1 differs from D₁ by a non-integral number of wavelengths. It is further desirable that the foregoing step of providing transducers having the above-described centerline spacing and the step of providing transducers having numbers of fingers that vary along the transducer sequence in the above-described manner, be carried out simultaneously as a part of the metallization, masking and etching steps to define the transducer conductor pattern on the substrate.

It is further desirable that the step of providing the transducer comprises, providing a number of fingers in a first input (or output) transducer located at an end of the transducer array which is larger than a number of fingers in another input (or output) transducer further along the sequence, and in further detail, providing a sequence of transducers symmetrical about a centerline and wherein the number of fingers in a first input (or output) transducer located near an end of the array is larger than the number of fingers in another input (or output) transducer located nearest the centerline.

In a preferred embodiment, the step of providing the transducers comprises providing one transducer in the sequence containing an odd number of electrode fingers and an adjacent transducer containing an even number of electrode fingers, and further that the step of providing the transducers comprises providing an input transducer with an even numbers of electrode fingers and an output transducer with an odd number of electrode fingers, or vice versa as regards the designations of input and output. In a still further embodiment, the step of providing the transducers comprises providing an input transducer with an even number of electrode fingers and another input transducer with an odd number of electrode fingers, and providing an output transducer with an odd number of electrode fingers, or vice versa as regards the designations of input and output.

Based on the foregoing description, it will be apparent to those of skill in the art that the present invention solves the problems and achieves the goals set forth earlier, and has substantial advantages as pointed out herein, namely, having larger numbers of electrode fingers on the outboard transducers compared to the inboard transducers improves the power handling capability. Further, the arrangement of the present design provides greater design flexibility in optimizing the impedance and spectral response.

While the present invention has been described in terms of particular materials, structures and steps, these choices are for convenience of explanation and not intended to be limiting and, as those of skill in the art will understand based on the description herein, the present invention applies to other choices of materials, arrangements and process steps, and it is intended to include in the claims that follow, these and other variations as will occur to those of skill in the art based on the present disclosure.